The effect of X-ray irradiation upon the epithelial melanin unit of the hair bulb in hooded rat

Circulating progenitor cells contribute to neointimal formation in nonirradiated chimeric mice

Recent evidence suggests that bone marrow-derived cells may contribute to repair and lesion formation following vascular injury. In most studies, bone marrow-derived cells were tracked by transplanting exogenous cells into bone marrow that had been compromised by irradiation. It remains to be determined whether endogenous circulating progenitors actually contribute to arterial remodeling under physiological conditions. Here, we established a parabiotic model in which two mice were conjoined subcutaneously without any vascular anastomosis. When wild-type mice were joined with transgenic mice that expressed green fluorescent protein (GFP) in all tissues, GFP-positive cells were detected not only in the peripheral blood but also in the bone marrow of the wild-type mice. The femoral arteries of the wild-type mice were mechanically injured by insertion of a large wire. At 4 wk, there was neointima hyperplasia that mainly consisted of alpha-smooth muscle actin-positive cells. GFP-positive cells were readily detected in the neointima (14.8+/-4.5%) and media (31.1+/-8.8%) of the injured artery. Some GFP-positive cells expressed alpha-smooth muscle actin or an endothelial cell marker. These results indicate that circulating progenitors contribute to re-endothelialization and neointimal formation after mechanical vascular injury even in nonirradiated mice.

Tyrosinase gene expression is regulated by p53

Tyrosinase, the rate-limiting enzyme for melanin synthesis, is induced after ultraviolet irradiation as part of the tanning response, the major recognized photoprotective response of human skin. Other DNA-damaging agents and DNA fragments such as thymidine dinucleotides also induce tyrosinase gene expression. Moreover, like ultraviolet light they also activate p53. To determine whether p53 activation is required for this increased tyrosinase expression, we employed two experimental systems: (i) a human melanoma line (WM35) known to express wild-type p53 versus WM35 cells engineered to express a transcriptionally inactive dominant-negative p53 (WM35-p53DN) or the empty vector alone (WM35-pCMV7) and (ii) mice with wild-type p53 versus p53 knockout mice. In WM35-p53DN cells, the baseline p53 protein level was higher than in WM35 or WM35-pCMV7 cells, and tyrosinase transcripts were lower. After ultraviolet irradiation, in all cell lines the p53 protein level increased within the first 24 h, as expected; and at 24 h tyrosinase mRNA levels were decreased. Consistent with the literature, these data in combination suggest that increased p53 protein level downregulates tyrosinase mRNA. In WM35 and WM35-pCMV7 cells at 48 and 72 h, however, whereas p53 levels remained elevated, tyrosinase mRNA levels compared to pre-irradiation levels tripled, whereas in WM35-p53DN cells levels remained below baseline. In thymidine-dinucleotide-treated WM35 and WM35-pCMV7 cells there was a comparable upregulation of tyrosinase mRNA within 24 h that persisted through 72 h, but there was no upregulation of tyrosinase mRNA in WM35-p53DN cells any time after ultraviolet irradiation or thymidine dinucleotide treatment. In ear skin of p53 wild-type mice, topical application of thymidine dinucleotide induced a 4-5-fold increase in epidermal melanin content after 3 wk, but in p53 knockout mice thymidine dinucleotide application caused no detectable increase in melanin. Together, these data demonstrate that p53 activation increases tyrosinase mRNA level and subsequently pigmentation. The data further suggest that tanning is part of a p53-mediated adaptive response of mammalian skin to DNA damage from ultraviolet irradiation.

Surface architecture of X-irradiated hair follicles

The combined use of scanning electron-microscopy and chemical separation of epidermis permitted the study of the irradiated and normal hair follicle surface characteristics. Irradiation reduced the length and density of the hair follicles, producing at the same time marked alterations and deformities of the individual follicles. As most of these changes were dose- and time-dependent, these structures, studied by these techniques, provide an interesting model as biological indicators of radiation dose.

Ultrastructural studies of epidermis in acute radiation dermatitis. Basal lamina thickening and coated vesicles

Fine structural changes of keratinocytes by X-ray were examined in normal skin area around a lesion of Bowen's disease. The area was exposed to about 6000 r over 28 days. The findings were: a decreased number of desmosomes and microvilli, formation of cytoplasmic vaculoes with or without membrane, perinuclear aggregation of tonofibrils, intracytoplasmic desmosomes and gap junction, cytoplasmic occurrence of dense bodies, lipid droplet and glycogen particles, changes in mitochondria, endoplasmic reticulum and Golgi complex, and deep invagination of the nuclear membrane. Besides these, conspicuous in the present study were, changes in the basal lamina which consisted of multiplication, thickening and occasional detachment from basal cells, and frequent occurrence of coated vesicles along with the basal lamina alteration. Possible exocytotic nature of coated vesicles in X-ray irradiated keratinocytes is discussed.